Various principles are known for accurately measuring distances between a subject, for example the surface of a clamped workpiece, and a reference point; these principles are used in interferometry, running-time methods, range-finding and triangulation (see T. C. Strand, "Optical Three-Dimensional Sensing for Machine Vision," Opt. Eng. 24, 33 (1985)). Since interferometry is not suitable for optically diffuse scattering surfaces, and running-time measurements are too inaccurate, most distance sensors used commercially are based on triangulation or range-finding. These two methods are physically similar and are also subject to the same physical limitations.
The most common distance sensor is based on the principle shown schematically in FIG. 1. According to it, a point of light 2 is projected onto a subject 4 as an image of a light source 1 in the direction of the axis of illumination 3. The coherent light reflected from the surface of the subject is collected in a measuring head 5 in the direction of the axis of observation 6, which is inclined at a triangulation angle .theta. to the axis of illumination on a point of light 7 in a photodetector 8. The measuring distance is found by triangulation. Thus, a change in the distance from the surface of the subject to the point of light 2 along a line segment .DELTA.z causes a measurable lateral movement .DELTA.x of the point of light 7 to position 9.
The accuracy of the distance determination is given by the accuracy in determining the point of light 7 and 9, but this has fundamentally physical limitations. The cause of these limitations is the statistical "speckle structure" of the point of light, as shown in FIG. 2. Speckle noises are basically created--even in light with so-called incoherent sources--because in the basic act of scattering, each excited atom emits light, whose phase is corrected with the phase of the exciting light, and the excited light and the scattered light are capable of interference or are "coherent with one another." This is also true when sunlight is scattered on subjects, just as it is with the known distance sensors. Because the coherence function becomes wider as the light spreads out, even the light of a light bulb is not completely spatially incoherent at the site where the surface of a subject is lighted, i.e., closely adjacent surface elements are lighted with their phases correlated. With coherent scattering, the diffuse reflected light of two adjacent points on a surface is almost always capable of interference.
The structure shown in FIG. 2 is determined by the respective microtopography of the surface of the subject and results in the fact that the respective point of light can only be detected with statistical uncertainty, which also affects the precision of the distance measurement. This connection and possible solutions are described in detail in DE-A-36 14 332, Wo 89/11 630 and DE-A-37 031 882.
It is also known (G. Hausler, "About fundamental limits of three-dimensional sensing," 15th Congress of the Int. Com. for Optics, Ed. F. Lanzl, 352 (1990)), that all methods that work with coherent light, run into the above-mentioned physical limit of accuracy, which is caused by the interaction of coherent light with the rough surfaces usual in mechanical engineering. The wave scattered on the subject has a phase that varies spatially and an intensity that fluctuates, which prevents localization of the scatter point above and beyond a certain limit of accuracy. This accuracy limit is basically given by the observation aperture: EQU .delta.x.apprxeq..lambda./sin u (1)
Where .delta.x stands for the statistical uncertainty (standard deviation) of the lateral localization of the point of light, .lambda. the wavelength and sin u the observation aperture. In triangulation measurements, between which an angle of triangulation .theta. is used between the axis of the beam and the axis of observation, the lateral localization uncertainty leads to a measurement uncertainty .delta.c regarding the distance: EQU .delta.c.apprxeq..lambda./(sin u.multidot.sin .theta.) (2)
In methods that are based on range-finding, the measuring uncertainty due to speckle is calculated by the equation (3): EQU .delta.c.apprxeq..lambda./sin.sup.2 u (3),
which corresponds to the classic Rayleigh field depth.
Equations (2) and (3) basically state that with coherent radiation, for a slight measuring uncertainty .delta.c, a large observation aperture sin u is necessary and/or a large angle of triangulation .theta.. For a measuring uncertainty that must be under 10 .mu.m, this leads to impractical angles .mu. and .theta.. For example, with a wavelength of .lambda.=0.8, in order to have a measuring accuracy of .delta.c=10 .mu.m for range-finding, an aperture of sin u.apprxeq.0.28 is needed, and hence an aperture angle of almost 30.degree.. Such an angle may be practical for microscopic applications, but it leads to very large lenses. And the field of vision interferes with the marginal beams, i.e., you cannot see into holes.
It is still essential to decide between accuracy and resolution: If you point one of the sensors described above at a subject, the sensor will show a distance that is accurate within the tolerance given by the equations (2), (3). If the subject is moved a little way toward the sensor, the corresponding change in distance is indicated. Hence, the sensor shows quite good distance resolution. But this is not true of subject movements that are small against the visual field of observation, since then only the speckle structure remains.
But if a subject with a completely plane (but rough) surface is moved parallel to the surface of the plane, the sensor should show the same distance, regardless of the movement. But this is not the case. The sensor signal simulates the presence of a "rocky" surface, as shown in FIG. 3, with a standard deviation in height that is given by equation (2) or (3). This standard deviation .delta.c is not the surface roughness of the subject, as is occasionally thought. .delta.c has nothing to do with the microtopography of the surface, but is determined exclusively by a property of the instrument, the observation aperture, and can be a multiple of the surface roughness of the subject.
In order to avoid the speckle structure and thus a statistical measurement error, the beam used for measurement, which codes the distance should be largely spatially incoherent. But this cannot be achieved by reproducing a (small) incoherent point source, for example, a high-pressure mercury sheet on the subject. Because, according to the coherence theory, after reproduction, the width of the spatial coherence function on the subject is equal to the width of the diffraction pattern (point image) of the lighting optic (M. Born, E. Wolf, "Principles of Optics," Pergamon Press, New York, 1970). Thus, the image of even a very small incoherent source is always spatially coherent.
Extensive spatial incoherence in the point of light 2 can therefore be achieved only when the radiation source 1 reproduced on the subject has a large surface so that the point of light is also large in relation to the point image of the projection lens. But this has the disadvantage that you are no longer addressing a small point on the subject by lighting, but rather a large area, so that the lateral resolution is reduced. Also, a point of light with a large surface can only be located with less accuracy on the photodetector. It was also attempted (see DE-A-36 14 332 and W. Dremel, G. Hausler, M. Maul, "Triangulation with Large Dynamic Range," Optical Techniques for Industrial Inspection, Ed. G. Paolo, Proc. SPIE 665, 182 (1986)) to light the subject with a laser and to move the laser point of light over the subject during measurement. This is equivalent to lighting with a large incoherent light source and leads to the same problems. Another possibility is to let the laser point of light dance statistically on the subject by producing turbulence in the aperture diaphragm of the lens being used. This is also equivalent to the method above and does not yield the required measurement accuracy in the .mu.m range. In other words, no spatially sufficient incoherent lighting can be achieved without the above-mentioned disadvantage of a large point of light. Temporally incoherent lighting is effective for speckle reduction only when the surface roughness of the subject is very large, which practically never exists in mechanical engineering.
The above-mentioned interference with high-accuracy distance determination by the "speckle structure" of the points of light obtained also exists with a measuring device known from DE-A-33 42 675 for optically scanning workpieces according to the triangulation principle. A point of light is projected by one or even two projectors at an angle to the surface of the measured subject, and the movement of the point of light that occurs when the distance from the subject changes is determined with a photosensitive detector. To increase the accuracy of the distance determination, besides the lateral movement of the image, the form and surface content of the point of light are measured by photoanalysis. But since the coherent radiation in the radiated point of light is analyzed, here again, measuring inaccuracies caused by the speckle structure of the point of light occur.
This also applies to the distance measurement devices known from DE-A-40 06 300 and U.S. Pat. No. 4,453,083.